For me- Sunset usually 2 stop (ND4). Here is a brief summary of when to use
Polarizer Filter: The polarizer filter helps enhance aireal videos by reducing the amount of light reflecting off the ground, sky, or water. Removing this reflecting light allows the Phantom 3 to capture beautifully saturated colors and improves contrast.
2-Stop Neutral Density: This 2-stop neutral density (ND) filter is the middle shade used for filming on mildly bright days i.e. party cloudy, or the golden hours. This filter helps remove highlights as well as reduces lens flare when shooting into the sun. This is the most commonly used of the two ND filters.
3-Stop Neutral Density: The 3-stop ND is the darkest shade and should be used on very bright days. This filter helps prevent lens flare, and reduces any mild rolling shutter. This filter also slows the shutter speed enough to blur the props on the Phantom 3 when they are in frame.